This invention relates generally to variable pumping methods and apparatus, and more particularly to low volume, low rate precision dispensing pumping apparatus.
There are many applications where precise control over the amount and/or rate at which a fluid is dispensed by pumping apparatus is necessary. In semiconductor processing, for example, it is important to control very precisely the amount and the rate at which photochemicals such as photoresist are applied to a semiconductor wafer being processed to manufacture semiconductor devices. The coatings applied to semiconductor wafers during processing typically require a flatness across the surface of the wafer that is measured in angstroms. Many processes today have requirements of the order 30 angstroms or less. The rate at which processing chemicals such as photoresists are applied to the wafer and spun out through centrifugal force to the edges of the wafer has to be controlled in order to ensure that the processing liquid is applied uniformly. It is also critical to control the rate and volume at which photoresist chemicals are applied to the wafer in order to reduce unnecessary waste and consumption. Many of the photochemicals used in the semiconductor industry today are not only toxic, but they are very expensive, frequently costing as much as $1,000 per liter. Thus, because of the cost of the chemicals as well as the difficulties in handling toxic materials, it is necessary to ensure that enough of the photoresist is applied to the wafer to satisfy processing requirements while minimizing excessive consumption and waste.
Another important requirement for semiconductor processing is the ability to repeatedly dispense very precisely a controlled amount of processing chemical each time, since variations in the amount of chemicals can adversely impact consistency from wafer to wafer. In the past, because of the unrepeatability as well as the inability to precisely control the amount of chemical being dispensed, many pumps had to dispense 50% to 100% more liquid than needed in order to ensure a sufficient quantity for processing requirements. This has resulted in waste and increased processing costs.
Some chemical dispensing pumps currently in use are known as half-step pumps. These pumps which are driven by a DC motor have the ability to step or rotate by a fixed angular amount. Each time the pump is advanced one step, it dispenses a predetermined amount of fluid. Typical half-stepping pumps use motors having approximately 200 sets of windings. Each winding corresponds to an angle of 360.degree./200=1.8.degree. per step. Half-step pumps energize the windings to produce fixed steps of 0.9.degree. corresponding to the angular positions of adjacent windings and midway between. With a conventional half-stepper pump, the step size and the corresponding amount of liquid dispensed per step are fixed. Therefore, the only way to control the total amount of liquid dispensed is to control the rate at which the pump is stepped. The step rate and step size produce a fixed volume per unit time. Typically, the dispense volume is relatively large in terms of the desired volume, so that the only way to control volume is to reduce the step rate. This has the effect of causing the chemicals to be dispensed as pulses of liquid, with the amount of dispense directly proportional to the angular size of the step, rather than as a smooth continuous flow of liquid. It also precludes precise control of the dispense volume which can adversely affect the quality of the resulting semiconductor products.
Other approaches which have been used to control chemical dispensing apparatus and the rate and quantity of the dispense are to employ mechanical devices such as valves and stops. However, this also does not permit the precise control desired for semiconductor processing operations.
It is desirable to provide low volume, low rate chemical dispensing pumping apparatus capable of precise and repeatable control of the rate and volume of chemicals dispensed by the pumping apparatus, and it is to these ends that the present invention is directed.